The present invention relates to metallurgical reactors, and more particularly so-called “smelter” metallurgical reactors suitably for carrying out a cast iron production process forming part of the group of processes known as “smelting reduction” processes. According to this group of processes, the cast iron is produced from: a material containing iron, for example iron ore and/or other reducible metal oxides such as manganese, nickel, chromium, etc., where applicable pre-heated and/or pre-reduced; a carbon-based reducing material, for example coal; a comburent gas containing oxygen, for example industrial oxygen. The products of the process are: liquid cast iron composed of an alloy of iron and other metals with a high concentration of carbon in solution form; the liquid slag, mainly composed of calcium, silicon, magnesium and aluminium oxides, and a gas containing sizeable fractions of carbon monoxide and carbon dioxide resulting from the reduction and combustion reactions.
The reactor according to the present invention is essentially composed of a metal casing internally lined, at least partially, with refractory material and provided, in the region of the top closure, with a duct through which the material containing iron or other reducible materials, for example iron ore, previously heated to a high temperature and partially reduced in a solid-state direct reduction reaction, for example a rotating-hearth furnace, is introduced.
In this metallurgical reactor it is required to perform efficient cooling of the ore supply duct both to protect it from the high temperatures and the damage resulting therefrom and to prevent adhesion, inside and outside thereof, of semi-molten materials and slag which would prevent the descent of the materials and would negatively affect regular execution of the process. The solution used in order to perform said cooling, which is known as “water jacket”, consists in surrounding this duct with a cavity inside which a cooling fluid flows. This solution may be regarded as being adopted from other metallurgical applications which are characterized by similar environmental conditions (for example oxygen lances for steel plant converters) where this problem is commonly solved by cooling, usually with water, the product which enters into the reactor.
One of the main problems in these reactors is that of ensuring both the regular descent of the charge material into the underlying slag bath and the elimination or reduction to a minimum of the material lost as a result of entrainment by the gases flowing out from the reactor.
In accordance with a main characteristic feature of the present invention, this problem is solved by providing, in the bottom terminal part of the said material loading duct, a series of nozzles for blowing in compressed gas, for example air, steam or nitrogen, in order to create a descending gaseous curtain around the charge material outflow opening, which assists regular descent of the said material, facilitating its introduction into the underlying liquid slag bath. Moreover, owing to the presence of these gaseous jets, in the vicinity of the outflow opening of the duct a dynamic vacuum is created, this vacuum counteracting any tendency of the process gas to rise back up through the duct during pressure transient peaks of the reactor due to the natural fluctuations in the process.
In accordance with a further feature of the present invention, the axis of the terminal part of the said material loading duct is advantageously inclined with respect to the vertical in the direction of the walls of the reactor and means are provided in order to rotate said duct part about a vertical axis so as to distribute the ferrous material the whole way around the chamber of the reactor, so as to prevent accumulation thereof in the central zone where there is greater turbulence, favouring at the same time introduction thereof into the underlying liquid slag bath.
The reduction smelting reactors of the type according to the invention are generally equipped with means for the injection of comburent gas, in some cases performed with lances which are suitably directed and arranged on at least two levels. In the reactor according to the present invention, via the lances positioned at a lower level (reducing zone), namely at the level of the reactor crucible, or via suitable lances positioned in the vicinity thereof, coal of suitable grain size is blown into the mass of molten cast iron by means of a suitable carrier gas.
The side walls and the bottom of the reactor are lined with refractory material suitable for containing the liquid phases of the process. To ensure efficiency of the process, an intense circulation of the liquid slag is required between the upper zone or oxidising zone and the bottom zone or reducing zone. This circulation obviously involves a high degree of heat exchange as a result of convection between the slag and the refractory lining which contains it. This, combined with the chemical aggressiveness of the liquid slag with respect to any refractory material with which it comes into contact, is a factor which greatly influences the duration of the refractory lining and, basically, in most of the already known smelting reduction processes is the main unresolved problem preventing commercialisation thereof.
In accordance with a further characteristic feature of the present invention, in order to overcome this problem, cooling elements are arranged in the wall section situated opposite the slag bath and the slag bath/cast iron transition zone, said elements being intended to remove the heat from the bath with an intensity such as to cause solidification of the slag and therefore prevent erosion of the refractory material, to a depth of penetration of said erosion, known as “freeze line”, of acceptable magnitude, namely sufficient for ensuring the structural stability of the remaining wall.
Advantageously, these cooling elements consist of plates made of metal with a high thermal conductivity, for example copper, formed preferably from a laminate in order to take advantage of the optimum mechanical properties and the improved thermal conductivity, compared to copper produced by means of casting, and consisting of solid metal on the inside of the casing and having formed in them channels through which the cooling fluid passes on the outside of the casing. The dimensions of these elements have been optimised in order to achieve various objectives: sufficient removal of heat in the specific slag turbulence conditions required by the process; keeping the temperature of the metal (copper) below the critical value for the long-term stability of its metallurgical properties; sufficient mechanical strength for interacting, without causing damage, with the surrounding refractory material during each operating stage, including the transient phases; easy replacement without the need to empty the reactor; suitable configuration for keeping the refractory material in position even when partly worn; lower weight (and consequently cost) per unit of surface area of the cooled wall; easy mechanical machining.
The top part of the reactor, above the liquid bath, is surrounded by cooled refractory or metallic walls and is closed at the top by a cooled metallic or refractory cover having formed in it an opening for outflow of the gases produced by the process and destined for processing and purification plants. The gas thus produced, which still contains a sizeable fraction of carbon monoxide, may be used, for example, as fuel in the pre-reduction rotating-hearth furnace.